How new research expands our understanding of the natural world

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Alfred Lord Tennyson puzzled over the conflict between love as a foundation of Christianity, and the apparent violence of the natural world.

Who trusted God was love indeed

And love Creation’s final law

Tho’ nature, red in tooth and claw

With ravine, shriek’d against his creed

The good poet would be relieved to learn that modern ecologists have uncovered a softer, gentler side of the natural world – facilitative interactions, in which one species (the facilitator) helps out a second species. In many, but not all, cases, the second species also helps out the first species. Ecologists describe these mutually-beneficial interactions as mutualisms. As an example, Mimosa luisana is a mutualist with Rhizobium bacteria, providing the bacteria with root nodules to live in and carbohydrates as an energy source, while receiving ammonia (NH3) that the bacteria fix (convert) from atmospheric N2. A second type of mutualism, a mycorrhizal association, is a very common facilitative interaction between plants and fungi, which grow alongside or within the plant roots. In many mycorrhizal associations, the plant provides carbohydrates to the fungi, which import and share nutrients and water.

Alicia Montesinos-Navarro and her colleagues, and researchers before them, noticed that in arid and semi-arid environments, plant-plant facilitation was most common between two plant species that were structurally and functionally very distinct, and that tended to be very distantly related to each other. In particular, M. luisana tends to associate with many different species of plants, including many cacti that look nothing like it, and are very distantly related. M. luisana is called a nurse plant, because other species tend to grow under its branches, which shade the soil and reduce water loss from evaporation. Recent work by Montesinos-Navarro and her colleagues showed another benefit of nursing – some plants receive nitrogen from these nurse plants via the network of mycorrhizal fungi.

Traditionally, ecologists have argued that associations between distantly-related plants occur because the plants have very different ecological niches, using different resources in different ways, so they are not competing with each other. Montesinos-Navarro and her colleagues argue that a second process might be important in this and other systems. Close relatives of M. luisana might tend to have high nitrogen levels and thus not benefit from nitrogen transfer from the nurse plant, while more distantly-related plants might tend to have lower nitrogen levels and thus benefit from any nitrogen arriving from M. luisana. They explored this hypothesis in the semi-arid Valley of Zapotitlan in the state of Puebla, Mexico.

Measuring nitrogen transfer from the nurse plant to the recipient is not the world’s easiest task. Fortunately there is a rare form or isotope of nitrogen, 15N, which can be distinguished from the more common 14N. The researchers soaked M. luisana leaves in urea that was made up of primarily 15N, and the leaves took up the urea. Consequently, any exported nitrogen would contain a disproportionately high concentration of 15N, resulting in high 15N levels in the recipient plant. They then measured 15N levels in 14 different species of plants that used M. luisana as their nurse. The researchers were able to test two hypotheses. First, they could see whether close relatives to M. luisana tended to have higher N-levels than more distantly related species. Second they could see whether distant relatives tended to receive more nitrogen from nurse plants than did close relatives.

The graph below summarizes the results. The y-axis measures how much the 15N level in the facilitated species increased by the end of the experiment (15 days). The x-axis measures the evolutionary relationship between M. luisana and the facilitated species – more precisely how long ago the two species shared a common ancestor. Lastly, the size of the dot measures the initial difference in leaf N-levels between M. luisana and the facilitated plant.

Influence of evolutionary relationship between M. luisana and the facilitated species (x- axis) and nitrogen gradient – the initial difference in nitrogen levels between the two species (size of dots) on the amount of nitrogen imported by the facilitated species.

Several trends are evident. First, close relatives of M. luisana tended to have similar leaf nitrogen values to M. luisana (medium sized dots), while distant relatives tended to have much less nitrogen than M. luisana (largest dots). Second, the most distant relatives tended to have the greatest increase in their 15N levels, which indicates that they received the greatest nitrogen export from their nurses.

One question is how the nitrogen is transported. Montesinos-Navarro and her colleagues describe how they treated soil with a fungicide, presumably killing the mycorrhizae, which resulted in a substantial reduction in nitrogen transport. This suggests that the mycorrhizal network is important for nitrogen transport. But more pressing is what do the nurse plants get out of the relationship. The researchers suggest that the recipient plants may provide M. luisana with either water or phosphorus, both of which may be in short supply in arid environments.

This study indicates that we need to look beyond traditional niche theory, and may need to dig deeper to understand the structure of plant communities, and how facilitative interactions can explain the coexistence of very distantly related plants.